A γ′-precipitation strengthened Ni-based superalloy is used as, for example, high temperature parts for a gas turbine or a steam turbine that requires mechanical strength under high temperature environment. It is said that the γ′-phase is composed of Ti, Al, Nb, and Ta and that a precipitation amount thereof can be increased by increasing a content of these constituent elements in the alloy and thereby mechanical strength of the alloy at high temperature can be enhanced.
On the other hand, in the case where the precipitation amount of the γ′-phase is made large so as to increase the mechanical strength of the alloy at high temperature, the hot forgeability (hot workability) of the alloy in the production process decreases and, if deformation resistance is thereby made excessively large, the forging itself cannot be performed in some cases. Particularly, it becomes a large problem in a large-sized product such as a turbine disk in which deformation by hot forging is unavoidable. Accordingly, a component composition of an Ni-based superalloy having both of the high-temperature strength and the hot forgeability has been investigated.
For example, Patent Document 1 discloses, as such an Ni-based superalloy, an alloy containing, in terms of % by mass, Al of from 1.3 to 2.8%, Co of from a minute amount to 11%, Cr of from 14 to 17%, Fe of from a minute amount to 12%, Mo of from 2 to 5%, Nb+Ta of from 0.5 to 2.5%, Ti of from 2.5 to 4.5%, W of from 1 to 4%, B of from 0.0030 to 0.030%, C of from a minute amount to 0.1%, and Zr of from 0.01 to 0.06%, in which, in terms of atomic %, (1) Al+Ti+Nb+Ta is from 8 to 11 and (2) (Ti+Nb+Ta)/Al is from 0.7 to 1.3. Therein, it is said that the total amount of Al, Ti, Nb, and Ta defines the solid solution temperature of the γ′ phase and the γ′ phase fraction, and according to the expression (1), the γ′ phase fraction is controlled within a range of from 30 to 44% and the solid solution temperature is controlled to lower than 1145° C. Furthermore, it is said that, according to the expression (2), the mechanical strength under high temperature environment owing to the γ′ phase is enhanced and also the precipitation of harmful n-type and δ-type needle-like intermetallic compound phases is prevented. It is said that according to the above, the alloy has such a high forgeability that cracking is not generated even in the forging at a temperature higher than the solid solution temperature of the γ′ phase, which is impossible in the case of UDIMET 720 (“UDIMET” is a registered trademark), and also said that the mechanical strength at 700° C. that is an operating temperature of a turbine can be increased as compared with the case of the Ni-based superalloy called 718 Plus.
Moreover, Patent Document 2 discloses an Ni-based superalloy having a component composition containing, in terms of % by mass, C of more than 0.001% and less than 0.100%, Cr of 11.0% or more and less than 19.0%, Co of 0.5% or more and less than 22.0%, Fe of 0.5% or more and less than 10.0%, Si of less than 0.1%, Mo of more than 2.0% and less than 5.0%, W of more than 1.0% and less than 5.0%, Mo+½W of 2.5% or more and less than 5.5%, S of less than 0.010%, Nb of 0.3% or more and less than 2.0%, Al of more than 3.00% and less than 6.50%, Ti of 0.20% or more and less than 2.49%, in which, in terms of atomic %, Ti/Al×10 is 0.2 or more and less than 4.0 and Al+Ti+Nb is 8.5% or more and less than 13.0%. Particularly, in Patent Document 2, the precipitation amount of the γ′ phase is increased by increasing the addition amount of Al, Ti, and Nb and, it is described that the high-temperature strength and the hot forgeability are in a trade-off relationship. In Patent Document 2, it is said that the content of Al is increased to prevent the solid solution temperature of the γ′ phase from rising and the high-temperature strength and the hot forgeability are both achieved. Therein, the content of Nb is controlled within a range of 0.3% or more and less than 2.0% and it is said that, in the case where Nb is contained in excess, the solid solution temperature of the γ′ phase rises to lower the forging workability and a Laves phase that is an embrittlement′ hase is generated to lower the high-temperature strength.
Patent Document 1: JP-T-2013-502511
Patent Document 2: JP-A-2015-129341